This invention relates to a method of preparing high emission efficiency thiogallate phosphors. More particularly, this invention relates to a method of preparing high emission efficiency alkaline earth metal thiogallate phosphors activated with europium.
Alkaline earth metal thiogallate phosphors (MGa2S4) activated with divalent europium, praseodymium, trivalent cerium and mixtures thereof, have been disclosed by Peters et al, J. Electrochem. Soc., Vol. 119, 1972, p230. These phosphors were made by solid state reaction from the alkaline earth sulfide, gallium sulfide and rare earth sulfides. They emit in the green to yellow region of the spectrum. They have good saturation properties, but their emission efficiency is low, at about 30% that of other sulfide phosphors. However, high efficiency phosphors are required for field emission displays, projection television, and blue-violet diode laser light sources.
Thus a method of preparing the above phosphors that results in an improvement of their emission efficiency would be highly desirable.
We have found that activated alkaline earth metal thiogallate phosphors having improved emission efficiency can be made by intimately mixing their insoluble sulfate salt precursors having a small particle size with gallium nitrate solution, in amounts to produce a small excess of gallium. The soluble salts are precipitated with sulfuric acid or ammonium sulfate to form their corresponding insoluble sulfates. These solids are then fired in hydrogen sulfide to form the corresponding thiogallate sulfide phosphors.
The present method includes the following steps.
A soluble alkali metal salt, as of strontium or calcium nitrate, is dissolved in dilute nitric acid. The desired amount of europium activator (1-6 mol percent) is added as a soluble salt, such as its nitrate. Neutralization with ammonium hydroxide produces a suspension of alkali metal sulfate particles coated with europium hydroxide.
The following equation summarizes this step:
Sr(SO4)+Eu(NO3)2+NH4OHxe2x86x92SrSO4.Eu(OH)3+NH4OH
Sulfuric acid or ammonium sulfate is added to precipitate the corresponding insoluble alkali metal sulfate. The particle size of the resultant precipitate should be kept small. This can be done by controlling the temperature and concentration of the soluble salt solution and by diluting the solution with an organic, miscible solvent, such as an alcohol.
A solution of an acid-soluble gallium salt, such as the nitrate, is also made. This can be done by dissolving the metal in nitric acid overnight. Since gallium oxide is very difficult to convert to an oxide-free sulfide with hydrogen sulfide, the oxide starting material is not recommended.
The gallium nitrate is added to the europium hydroxide coated alkaline earth sulfate in sufficient amount so as to produce an excess of from about 0.1-7 percent by weight of gallium in the final gallium sulfide product. The phosphor precursor is shown below, where xe2x80x9c2.01xe2x80x9d indicates a slight excess of the gallium hydroxide, as
Sr(SO4):Eu:2.01Ga(OH)3
After combining these solutions and bringing the mixture to a neutral pH with ammonia, or by carrying out a precipitation of gallium using urea, a solid precipitates. The solid phosphor precursor is dried, ground, placed in a refractory boat, such as an alumina boat, and fired in hydrogen sulfide for about five hours in a tube furnace. Suitably the firing temperature is about 800xc2x0 C. The sulfide product obtained is shown below:
SrGa2S4:Eu:Ga2S4
To ensure uniformity of the product, the sulfide material is ground to a powder and refired in hydrogen sulfide at 900xc2x0 C. for about two hours. The resultant strontium thiogallate phosphor has a particle size of about 8-10 microns. Its emission efficiency was measured at 80-100 percent.
The resultant green emitting, high emission efficiency phosphor should show a slight excess of gallium sulfide by means of x-ray analysis, in the range of about 0.5-7%.
If an organic solvent, such as an alcohol or acetone is added when the sulfate is precipitated, and firing is carried out at a lower temperature, such as 780xc2x0 C. for five hours and 850xc2x0 C. for about four hours, the average particle size of the product will be somewhat smaller, e.g., about 5-6 microns. Thus the product particle size can be controlled as required by the final use of the phosphor.
A like calcium gallium sulfide activated with Eu can be made in similar manner but substituting a calcium salt for the strontium salt. A mixed crystal phosphor of strontium and calcium can also be made. Cerium or praseodymium can be substituted, in whole or in part, for the europium activator.
Reducing the amount of gallium present (for example a 5 to 3% excess) reduces the particle size and sometimes the relative efficiency of the SrGa2S4 phosphor.
For example, samples with a 5% excess gallium, fired at 900xc2x0 C. for two hours can have a relative efficiency range of from about 66 to 100%, with a median particle size of about 8.59 to 9.41 microns.
A sample with 3% excess gallium, plus the addition of alcohol in the precipitation step and fired at 850xc2x0 C. for four hours, can have a relative efficiency of about 82% and a median particle size of about 3.63 microns.
The invention will be further described by means of the following examples. However, the invention is not meant to be limited to the details described therein.
In the Examples, all parts are by weight.